Silica sol

Abstract

The present invention relates to a silica sol having a pH above about 10, molar ratio of SiO2:M2O, in which M is alkali metal, of from about 6:1 to about 16:1, concentration of soluble silica above about 3000 mg SiO2/l, and S-value of from about 18 to about 40%. The invention further relates to a process for producing a silica sol which comprises acidifying an aqueous alkali metal silicate solution to a pH of from 1 to 4 to form an acid sol, alkalizing the acid sol by addition of aqueous alkali metal silicate solution to obtain an intermediate sol and interrupt the addition of aqueous alkali metal silicate solution when the intermediate sol has reached a pH of from about 5 to about 8, stirring the intermediate sol for a time period of from about 10 to about 6000 seconds without addition of aqueous alkali metal silicate solution, alkalizing the intermediate sol by addition of aqueous alkali metal silicate solution to obtain a silica sol having a pH above about 10 and molar ratio of SiO2:M2O, in which M is alkali metal, of from about 6:1 to about 16:1, and optionally adding an aluminum compound to the acid sol, intermediate sol or silica sol. The invention further relates to a silica sol obtainable by the process of the invention. The invention further relates to the use of the silica sol as a flocculating agent, in water purification and in producing paper and board. The invention further relates to a process for producing paper and board which comprises: (i) providing an aqueous suspension comprising cellulosic fibers; (ii) adding to the suspension one or more drainage and retention aids comprising the silica sol; and (iii) dewatering the obtained suspension to provide a sheet or web of paper or board.

The invention claimed is: 
     
       1. A process for producing a silica sol, the process comprising,
 (a) acidifying an aqueous alkali metal silicate solution to a pH of from 1 to 4 to form an acid sol, 
 (b) alkalizing the acid sol by addition of aqueous alkali metal silicate solution to obtain an intermediate sol and interrupt the addition of aqueous alkali metal silicate solution when the intermediate sol has reached a pH of from about 5 to about 8, 
 (c) stirring the intermediate sol for a time period of from about 10 to about 6000 seconds without addition of aqueous alkali metal silicate solution, and 
 (d) alkalizing the intermediate sol by addition of aqueous alkali metal silicate solution to obtain a silica sol having a pH above about 10 and molar ratio of SiO 2 :M 2 O, in which M is alkali metal, of from about 6:1 to about 16:1, 
 (e) optionally adding an aluminum compound to the acid sol, intermediate sol or silica sol. 
 
     
     
       2. The process according to  claim 1 , wherein the process comprises adding an aluminum compound to the acid sol, intermediate sol or silica sol. 
     
     
       3. The process according to  claim 1 , wherein the aluminum compound is added in an amount to obtain a silica sol having a molar ratio of Si:Al of from about 25:1 to about 250:1. 
     
     
       4. The process according to  claim 1 , wherein the silica sol has a concentration of soluble silica above about 3000 mg SiO 2 /1. 
     
     
       5. The process according to  claim 4 , wherein the silica sol has an S-value of from about 18 to about 40%. 
     
     
       6. The process according to  claim 1 , wherein the silica sol has an S-value of from about 18 to about 40%. 
     
     
       7. The process according to  claim 1 , wherein the sol contains silica particles having a specific surface area of at least about 500 m 2 /g. 
     
     
       8. The process according to  claim 1 , wherein the silica sol has a silica content of from about 3 to about 20% by weight. 
     
     
       9. The process according to  claim 1 , wherein the silica sol has a pH of from about 10.5 to about 11.5. 
     
     
       10. The process according to  claim 1 , wherein the silica sol has a molar ratio of SiO 2 :M 2 O, in which M is alkali metal, of from about 8:1 to about 15:1. 
     
     
       11. The process according to  claim 1 , wherein the silica sol has a concentration of soluble silica of at least about 3500 mg SiO 2 /l. 
     
     
       12. The process according to  claim 1 , wherein the silica sol has a conductivity of at least about 4.0 mS/cm. 
     
     
       13. A method of flocculation, comprising performing the process of  claim 1 , and providing the silica sol so obtained as a flocculating agent. 
     
     
       14. The process according to  claim 13 , wherein the silica sol has an S-value of at least 20%. 
     
     
       15. A process for producing paper and board comprising,
 (i) preparing a silica sol according to the process of  claim 1 ; 
 (ii) adding one or more drainage and retention aids to an aqueous suspension comprising cellulosic fibers in which one of the drainage and retention aids is the silica sol from (i); and 
 (iii) dewatering the obtained suspension to provide a sheet or web of paper board. 
 
     
     
       16. The process according to  claim 15 , wherein said one or more drainage and retention aids comprise a cationic polymer. 
     
     
       17. The process according to  claim 15 , wherein said one or more drainage and retention aids comprise an anionic polymer. 
     
     
       18. The process according to  claim 15 , further comprising adding to the suspension an aluminum compound.